Substituent

About: Substituent is a research topic. Over the lifetime, 42877 publications have been published within this topic receiving 516716 citations. The topic is also known as: side chain & side group.

Structures and Chemistry of Methanofullerenes: A Versatile Route into N-[(Methanofullerene)carbonyl]-Substituted Amino Acids

Abstract: The reaction of C60 with oxadiazole 13 afforded the dimethoxymethanofullerene 7 in 32% yield as a 6-6-ring-bridged isomer with a closed transannular bond. A literature survey showed that all 6-6-ring-bridged methanofullerenes are σ-homoaromatic with a closed transannular bond (6-6-closed) and all 6-5-ring-bridged are π-homoaromatic with an open transannular bond (6-5-open). The preference for 6-6-closed and 6-5-open structures is not due to substituent effects but is best explained with the conservation in these isomers of the favorable bonding seen in C60 with higher double-bond character at 6-6 bonds and higher single-bond character at 6-5 bonds. Reaction of C60 with diazo diester 15 gave the fullerene diester 14 which was hydrolyzed with BBr3 in benzene to the methanofullerenecarboxylic acid 10, a versatile synthon for the preparation of amphiphilic fullerene derivatives. Treatment of 10 with alcohols and amino acid esters under DCC coupling conditions afforded the esters 5 and 17 and the amino-acid derivatives 11 and 12, respectively.

Identification of the products of oxidation of quercetin by air oxygen at ambient temperature.

Abstract: Oxidation of quercetin by air oxygen takes place in water and aqueous ethanol solutions under mild conditions, namely in moderately-basic media (pH approximately 8-10) at ambient temperature and in the absence of any radical initiators, without enzymatic catalysis or irradiation of the reaction media by light. The principal reaction products are typical of other oxidative degradation processes of quercetin, namely 3,4-dihydroxy-benzoic (proto-catechuic) and 2,4,6-trihydroxybenzoic (phloroglucinic) acids, as well as the decarboxylation product of the latter--1,3,5-trihydroxybenzene (phloroglucinol). In accordance with the literature data, this process involves the cleavage of the gamma-pyrone fragment (ring C) of the quercetin molecule by oxygen, with primary formation of 4,6-dihydroxy-2-(3,4-dihydroxybenzoyloxy)benzoic acid (depside). However under such mild conditions the accepted mechanism of this reaction (oxidative decarbonylation with formation of carbon monoxide, CO) should be reconsidered as preferably an oxidative decarboxylation with formation of carbon dioxide, CO2. Direct head-space analysis of the gaseous components formed during quercetin oxidation in aqueous solution at ambient temperature indicates that the ratio of carbon dioxide/carbon monoxide in the gas phase after acidification of the reaction media is ca. 96:4%. Oxidation under these mild conditions is typical for other flavonols having OH groups at C3 (e.g., kaempferol), but it is completely suppressed if this hydroxyl group is substituted by a glycoside fragment (as in rutin), or a methyl substituent. An alternative oxidation mechanism involving the direct cleavage of the C2-C3 bond in the diketo-tautomer of quercetin is proposed.

Aromatic hydroxylation by cytochrome P450: model calculations of mechanism and substituent effects.

Abstract: The mechanism and selectivity of aromatic hydroxylation by cytochrome P450 enzymes is explored using new B3LYP density functional theory computations. The calculations, using a realistic porphyrin model system, show that rate-determining addition of compound I to an aromatic carbon atom proceeds via a transition state with partial radical and cationic character. Reactivity is shown to depend strongly on ring substituents, with both electron-withdrawing and -donating groups strongly decreasing the addition barrier in the para position, and it is shown that the calculated barrier heights can be reproduced by a new dual-parameter equation based on radical and cationic Hammett sigma parameters.

Imidazolidine-based metal carbene metathesis catalysts

Abstract: The present invention relates to novel metathesis catalysts with an imidazolidine-based ligand and to methods for making and using the same. The inventive catalysts are of formula (I) wherein : M is ruthemium or osmium; X and X1 are each independently an anionic ligand; L is a neutral electron donor ligand; and, R, R?1, R6, R7, R8 and R9? are each independently hydrogen or a substituent selected from the group consisting of C?1?-C20 alkyl, C2-C20 alkenyl, C2-C20 alkynyl, aryl, C1-C20 carboxylate, C1-C20 alkoxy, C2-C20 alkenyloxy, C2-C20 alkynyloxy, aryloxy, C2-C20 alkoxycarbonyl, C1-C20 alkylthiol, aryl thiol, C1-C20 alkylsulfonyl and C1-C20 alkylsulfinyl, the substituent optionally substituted with one or more moieties selected from the group consisting of C1-C10 alkyl, C1-C10 alkoxy, aryl, and a functional group selected from the group consisting of hydroxyl, thiol thioether, ketone, aldehyde, ester, ether, amine, imine, amide, nitro, carboxylic acid, disulfide, carbonate, isocyanate, carbodiimide, carboalkoxy, carbamate, and halogen. The inclusion of an imidazolidine ligand to the previously described ruthenium or osmium catalysts has been found to dramatically improve the properties of theses complexes. The inventive catalysts maintains the functional group tolerance of previously described ruthenium complexes while having enhanced metathesis activity that compares favorably to prior art tungsten and molybdenum systems.